1. Field of the Invention
The disclosure relates generally to a sensor for measuring biosignals such as Entropy, EEG, EKG, EMG; and more specifically to sensors which comprise at least one electrode comprising a substrate of flexible nonconductive material, a conductive layer, a gel layer and a barrier layer.
2. Description of Related Art
Electrodes which are used to record biosignals from the recording surface, for example the skin, generally require the use of a conductive liquid or solid gel to provide a continuous conductive path between the recording surface and the electrode sensing element. Conductive gels contain a salt, KCl or NaCl, in order to achieve electrical current flow. The preferred gel is one with a high salt content, since such a gel produces a better conductor than that obtained when using a gel with low salt content. In addition, the use of a high salt content typically requires less skin abrasion at the time of application to reduce the impedance of the skin-electrode interface after subsequent electrode application.
Biosignal measurement electrodes can be single electrodes or electrode arrays containing multiple electrodes at the same substrate. Electrodes typically contain an adhesive foam material that is used for attaching the electrode to living tissue, for example, a human forehead or chest depending on the use area. Electrode contains electrolyte gel with salt content that is in direct contact with tissue to enable measurement of the electrical signal. The typical use time of an electrode is dependent on the application and varies from minutes to several days.
Biosignal measurement sensor electrodes with high salt content traditionally have a 12 month shelf life. This is caused by many factors, for example, drying of the gel in the electrodes, but mainly by the changes that take place in the conductive layer and the barrier layer. The conductive layer can be, for example, silver (Ag) and the barrier layer can be, for example, silver/silver-chloride (Ag/AgCl). The layers are placed contiguously on top (tissue side) of each other. The changes are caused by the chemical reactions between the layers and electrolyte gels that have high salt content to maximize the signal quality and low impedance.
Traditionally as shown in FIG. 1 the barrier layer 3, for example, Silver/Silver Chloride (Ag/AgCl) is placed directly on the top (tissue side) of the conductive layer 2, for example, Silver (Ag). The gel 5 is placed directly on the top (tissue side) of the thin layer such as Ag/AgCl that acts as a barrier layer 3 between the conductive layer 2 such as Ag and the gel 5. This barrier layer 3 is very thin, usually from a few to tens of micrometers, and the gel 5 changes the features of the layer over time. If the gel contacts the Ag layer directly after a period of time, the performance of the Ag layer and the whole electrode deteriorates.
Another method known in the field to create a barrier layer is to add an active gel on the plain top surface (tissue side) of the conductive Ag layer. The gel modifies the Ag layer and chemically changes the top surface (tissue side) to Silver Chloride Ag/AgCl. The chemical reaction is controlled by adding a controlled amount of a substance to the plain silver that stops the chemical reaction at a defined point. However, this method creates a really thin AgCl layer on top (tissue side) of the Ag layer where the gel penetrating through the thin AgCl layer to Ag layer can lower the shelf life of the electrode.
The basic methods to manufacture such electrodes are well known in the field. These methods are used widely for manufacturing printed electronics. These methods are for example silk-screen printing, flexography, gravure, offset lithography and inkjet. All of these methods use printable inks such as silver (Ag) and silver/silver chloride (Ag/AgCl) that can be deposited on the flexible substrate in automated process enabling mass production of described sensors. A person who is skilled in the art can find various other techniques that can be used for manufacturing described electrode embodiment.
The disclosure provides a sensor for measuring biosignals that can avoid the limitations of the prior art.